I have all the requirements for Orc fortress walls. Scroll of Building, 3,800 food, 12,000 gold, wood and 60 crowns. The button to purchase fortress walls is grayed out. I don't have palisade walls yet. Do I have to get palisade walls before fortress? If I do I find that really dumb as they will be knocked down by the fortress walls anyway. The descriptions for getting certain things are quite vague in this game and its getting frustrating.